1. Field of the Invention
The invention relates generally to Global Positioning System receivers and more particularly to a Global Positioning System receiver having a rapid acquisition of GPS satellite signals.
2. Background of the Invention
A Global Positioning System (GPS) now provides a worldwide, 24 hour, location service. The system includes multiple GPS satellites to broadcast location signals, control stations to monitor and control the satellites, and GPS receivers to receive the signals. Commercial Global Positioning System (GPS) receivers now are used to provide accurate location information in many navigation, tracking, and timing applications. A GPS antenna that is a part of a GPS receiver must have a line of sight to a GPS satellite to receive the GPS signal from that satellite.
GPS location is based on one-way ranging from the GPS satellites to the GPS antenna. Ranges are measured to four satellites simultaneously in view by matching (correlating) the frequency and the time of arrival (TOA) of the incoming GPS signal to a receiver-generated replica signal. With four ranges, the receiver can determine four unknowns, typically latitude, longitude, altitude, and an adjustment to the replica. The ranges are called "pseudoranges" because the actual distances to the GPS satellite are not known until the internal replica has been adjusted. Time of day is computed from the adjustment to the TOA of the replica.
Each GPS satellite broadcasts its position in a signal having a carrier frequency at approximately 1.575 GHz. The signal is modulated by a PRN code sequence of 1023 chips, repeating (arriving) at a 1 millisecond time interval, where each satellite uses a different PRN code sequence. The use of the different PRN sequences enables a GPS receiver to distinguish the GPS signals from different GPS satellites. The frequency of the signal received from each GPS satellite will have a Doppler shift due to the relative velocity between the GPS satellite and a GPS antenna. A velocity for the GPS antenna may be determined from the rate of change of the location or from the rate of change of the pseudoranges after accounting for the Doppler shift due to the motion of the satellite.
Power consumption is an important figure of merit for a GPS receiver. A low power consumption is good for a GPS receiver that depends upon a battery for a power source. To achieve low power, some GPS receivers have a low power standby mode, where power consumption is reduced but the GPS signals are not tracked. Some GPS receivers have a system to alternate between a normal mode to obtain a location fix and the low power standby mode.
Another important figure of merit for a GPS receiver is the time required to obtain a location fix or the "acquisition time." A fast acquisition is good because a user does not need to wait as long for a new location fix. In GPS receivers that automatically cycle to power off or to the standby mode between location fixes, a fast acquisition time is good because less time is used in the normal mode to acquire the GPS signal, resulting in a lower average power consumption.
Three acquisition times are often quoted. A "time to first fix," sometimes called a "cold start" acquisition, refers to the time to acquire the GPS satellite signal and obtain a location fix when the GPS receiver has not had a location fix within the previous few hours. A "signal interruption" acquisition time refers to the time to reacquire the GPS satellite signal and to obtain a location fix after the line of sight to the GPS satellite signal has been blocked and then unblocked. A "time to subsequent acquisition," sometimes called a "warm start" acquisition, refers to the time to reacquire the GPS satellite signal and obtain a location fix, when the GPS receiver has had a location fix within the previous few hours.
The subsequent acquisition is fast when the initial replica frequency is within 500 Hertz (0.3 parts per million) of the GPS frequency and the initial replica TOA is within 500 .mu.s of the GPS TOA. Typically, the greater the differences between the initial replica frequency and the GPS frequency and between the initial replica TOA and the GPS TOA, the greater the time required to acquire the GPS signal.
Some GPS receivers reduce the difference between the initial replica frequency and the GPS frequency by synchronizing the replica frequency to a reference oscillator that continues to operate during a standby time duration. However, a change in temperature inside the receiver, because less heat is being generated during the standby mode or/and because the outside temperature changes, will probably cause the reference oscillator frequency to drift, thereby causing the replica frequency to drift. An inexpensive XO reference oscillator, using a crystal as a resonator, provides a frequency stability of approximately 5 to 50 parts per million (ppm) within a temperature range of -40.degree. C. to +85.degree. C. A problem with an XO is that a change in temperature of 1.degree. C. or less typically causes a change in frequency of more than 0.3 ppm. A temperature compensated crystal oscillator (TCXO) may be used to improve the stability to approximately 0.5 to 5 ppm within -40.degree. C. to +85.degree. C. but still may not be stable enough to provide 0.3 ppm during a typical standby time duration. A further problem with a TCXO is that the incremental cost of a TCXO over an XO is a significant part of the cost of an entire GPS receiver. A temperature stabilized oven, enclosing the reference oscillator, further improves the frequency stability but the oven would have to remain on, requiting a large power consumption during the standby mode in order to provide the improvement. A further problem is that an oven stabilized oscillator may cost as much or more than an entire GPS receiver.
What is needed is a GPS receiver having a rapid acquisition of GPS satellite signals, after a time duration in a low power standby mode, using a reference oscillator having a frequency variation of up to 50 ppm over the range of -40.degree. C. to +85.degree. C.